1. Field of the Invention
The present invention relates to a thermal printing method and a thermal printer, and more particularly to a thermal printing method and a thermal printer capable of expressing tonal steps larger than that in which a head driver can express inherently.
2. Description of the Background Art
Sublimation transfer type thermal printers and direct imaging type thermal printers have been known, the former heating an ink film with a thermal head and transferring ink to a recording sheet and the latter directly heating a thermosensitive recording sheet with a thermal head to develop color on the sheet. Both types of thermal printers use a thermal head having a plurality of heating elements disposed in a main scan direction, and each heating element is heated to record an image on a recording sheet one line after another, while moving the thermal head or the recording sheet in a sub-scan direction. The heating energy of each heating element is controlled in accordance with heating data of each dot to change the density of the dot to be formed on the recording sheet and express a tonal level of the pixel.
For example, a color thermal printer uses a color thermosensitive recording sheet which has cyan, magenta, and yellow thermosensitive coloring layers, and a protective layer laminated on a supporting layer in this order. Each thermosensitive coloring layer has a different heat energy to develop color, and the deeper layer requires a larger heat energy. Each thermosensitive coloring layer is selectively colored depending upon the different heat energy for coloring from other thermosensitive coloring layers. Prior to recording the next thermosensitive coloring layer, a specific ultraviolet ray is applied to the already recorded thermosensitive coloring layer to fix it so as not to develop color again. In this manner, thermosensitive coloring layers of three colors are sequentially recorded to form a full-color image on a color thermosensitive recording sheet.
In printing one dot on a thermosensitive coloring layer, each heating element supplies the color thermosensitive recording sheet with a heat energy immediately before coloring (hereinafter called a bias heating energy) to perform bias heating. After this bias heating, a heat energy for coloring at a desired density (hereinafter called an image heating energy) is applied to the thermosensitive recording sheet to perform image heating. With these bias heating and image heating, a square pixel area virtually partitioned on the color thermosensitive recording sheet is colored to record one dot.
A thermal head has a heating element array constituted by a number of heating elements disposed in line and a head driver for controlling the heat energy of each heating element in accordance with its heating data. For the bias heating, bias data is used as the heating data, and for the image heating, image data is used as the heating data.
The head driver has a comparator which compares the heating data with sequentially counted-up comparison data to turn the heating element on or off in accordance with the comparison result. The head driver converts the heating data into a train of drive pulses corresponding in number to the value of the heating data. This drive pulse train intermittently drives each heating element of the thermal head. Each heating element may be driven for a time duration corresponding to the heating data.
For example, in recording a half-tone image having "256" tonal levels, an 8-bit thermal head is used to supply the heating data of 8 bits per one pixel to the head driver which converts the 8-bit heating data into a train of serial drive pulses of 0 to 255. This serial drive pulse train drives each heating element 0 to 255 times to change the heat energy 256 steps.
If the number of bits of the heating data is increased to increase the number of tonal levels, a conventionally used thermal head cannot be used. For example, an 8-bit thermal head widely used nowadays cannot record a half-tone image, for example, if using 9-bit heating data to obtain 511 tonal levels.